1. Field of the Invention
The present invention relates to a respiratory mask and a vent for a respiratory mask.
2. General Background and Related Art
The application of Continuous Positive Airway Pressure (CPAP) via a nasal mask is a common ameliorative treatment for sleep disordered breathing (SDB) including obstructive sleep apnea (OSA) as described in the applicant's U.S. Pat. No. 4,944,310. In CPAP treatment for OSA, air or other breathable gas is supplied to the entrance of a patient's airways at a pressure elevated above atmospheric pressure, typically in the range 3-20 cm H2O as measured in the patient interface. It is also known for the level of treatment pressure to vary during a period of treatment in accordance with patient need, that form of CPAP being known as automatically adjusting nasal CPAP treatment as described in the applicant's U.S. Pat. No. 5,245,995.
Non-invasive positive pressure ventilation (NIPPV) is another form of treatment for breathing disorders including sleep disorder breathing. In a basic form NIPPV involves a relatively high pressure of gas being provided in the patient interface during the inspiratory phase of respiration and a relatively low pressure or atmospheric pressure being provided in the patient interface during the expiratory phase of respiration. In other NIPPV modes the pressure can be made to vary in a complex manner throughout the respiratory cycle. For example, the pressure at the patient interface during inspiration or expiration can be varied through the period of treatment as disclosed in the applicant's International PCT Patent Application No WO 98/12965 and International PCT Patent Application No WO 99/61088.
In this specification any reference to CPAP treatment is to be understood as embracing all of the above-described forms of ventilatory treatment or assistance.
Typically, the patient interface for CPAP treatment consists of a nasal mask. The nasal mask is generally defined by a mask shell which forms an inner cavity defined by its interior surface, mask cushion and the user's face, a gas inlet which may or may not include a separate component such as a swivel elbow. Alternatively, a nose-mouth mask or full-face mask or nasal prongs or nasal pillows can be used. In this specification any reference to a mask is to be understood as incorporating a reference to a nasal mask, nose-mouth mask, full face mask, nasal prongs or nasal pillows unless otherwise specifically indicated. The mask incorporates, or has in close proximity, a gas washout vent for venting exhaled gases to atmosphere. The gas washout vent (the vent) is sometimes referred to as a CO2 washout vent.
It is important that the apparatus is quiet and comfortable to encourage patient compliance with therapy. The exhausting to atmosphere of exhaled gas through the vent creates noise. As CPAP and NIPPV treatments are normally administered while the patient is sleeping, minimization of such noise is desirable for both the comfort of the patient and any bed partner.
From a clinical perspective it is desirable for a mask and vent combination to maximize both the elimination of exhaled CO2 through the vent and also the inhalation of the supplied breathable gas. In this way, retention of exhaled CO2 within the mask, which is “re-breathed” by the wearer, is minimized. Generally by locating the vent in the mask shell CO2 washout will be superior to locating the same vent between the mask shell and the breathable gas supply conduit.
It is desirable to minimise the weight of the vent assembly for greater patient comfort.
Systems for the delivery of nasal CPAP treatment often incorporate in-line humidifiers to minimize drying of the nasal mucosa and increase patient comfort. Accordingly, it is also desirable that a vent not block when used with humidified gas. It is also desirable that a vent be easily cleaned or economically disposable.
A number of vent configurations are known. One approach to vent configuration is to create within the mask shell one or more openings that allow for the flow of exhaust gas from the inner cavity to atmosphere. The exhaust flow may be directed through the incorporation of an additional pipe extending out from the opening located on the mask shell outer surface.
The applicant's nasal mask system known by the name ResMed Modular Mask System incorporates an outlet vent located in the swivel elbow connected to the mask shell. The ports defining the vent have the same cross-sectional thickness and are formed from the same polycarbonate material that is used to form the swivel elbow and mask shell frame.
The whisper swivel, manufactured by Respironics, Inc provides three slots on the circumference of a generally cylindrical attachment piece. In use, the attachment piece is to be interposed between the mask shell and the gas conduit. The attachment piece is made of the same material and thickness as is used to make the mask shell.
European Patent No. 0 697 225 discloses a vent formed from a porous sintered material.
A known vent, manufactured by Gottleib Weinmann Gerate Fur Medizin Und Arbeitsschutz GmbH and Co. comprises a generally cylindrical insert to be interposed in use, between the mask shell and the gas conduit. The insert includes a window which is covered with a porous sintered material of approximately 3-4 mm thickness.
Another type of vent intended to be inserted between the mask shell and the breathable gas supply conduit is the E-Vent N by Draeger medizintechnik GmbH (the Draeger vent). The Draeger vent comprises a stack of 21 annular disks, which have slots in their adjacent surfaces for gas to flow therethough. Each slot has a length of 5 to 7 mm as measured along the path from the interior of the vent to atmosphere.
The applicant produces a respiratory mask known as the MIRAGE® nasal mask system and the MIRAGE® full-face mask (the MIRAGE mask). The MIRAGE® mask has a crescent shaped opening in the mask shell in which is located a complementary shaped crescent elastometric insert with six holes therein which constitutes the vent. The elastomeric inset has a crossectional thickness of 3 to 4 mm. The vent of the type used in the MIRAGE® is described in International Patent Application No. WO 98/34665 and Australian Patent No 712236.
It is an object of the present invention to provide an alternative form of vent that is suitable for use in a respiratory mask.